This invention relates to an improved fibrous mat-faced gypsum board, for example, gypsum board faced with glass fiber mat. More particularly, the present invention relates to a gypsum board faced with a coated glass fiber mat. The coating comprises a dried aqueous mixture of a mineral pigment; a first binder comprised of a polymer latex adhesive; and, a second binder comprised of an inorganic adhesive.
The present invention is particularly advantageous for use in any application in which the fibrous mat-faced gypsum board is expected to be exposed to a high humidity or high moisture environment during installation or use, such as in shaft walls, stairwells, area separation walls, return air installations and as a tile backer in bathroom applications. Still other applications and uses will become apparent from the detailed description of the invention, which appears hereinafter.
Panels of gypsum wallboard which comprise a core of set gypsum sandwiched between two sheets of facing paper have long been used as structural members in the fabrication of buildings where the panels are used to form the partitions or walls of rooms, elevator shafts, stairwells, ceilings and the like. A specialty application for the use of panels of gypsum wallboard, as well as other types of building panels, is the use thereof in bathroomsxe2x80x94typically a place of high humidity and residual water because of the flow of water from the use of showers, bathtubs, and sinks. Gypsum wallboards suitable for use in these applications share a common requirement; that is a resistance or tolerance to high humidity and high moisture environments, often for prolonged periods.
A usual construction of bathroom walls includes a multi-ply structure of ceramic tile adhered to an underlying base member, for example, a panel of wallboard comprising gypsum or other material as will be described below. Such a panel is referred to in the industry as a xe2x80x9ctile backing board,xe2x80x9d which for convenience is referred to herein as xe2x80x9ctile backerxe2x80x9d. In usual fashion, sheets of tile backer (for example, 4xe2x80x2xc3x978xe2x80x2xc3x97xc2xdxe2x80x3) are fastened by rust-resistant nails or screws to studs. Blocks of ceramic tiles (for example, 4xe2x80x3xc3x974xe2x80x3) are adhered to the sheets of tile backer by water-resistant adhesive which is referred to in the industry as xe2x80x9cmasticxe2x80x9d or by a Portland cement-based adhesive which is referred to commonly as xe2x80x9cthin set mortarxe2x80x9d. Thereafter, spaces between the tiles and between the tiles and other adjoining surfaces, for example, the lip of a bathtub or sink, are filled with a water-resistant material which is referred to in the industry as xe2x80x9cgroutingxe2x80x9d.
It should be appreciated that a primary goal in constructing a bathroom that includes one or more of a bathtub, shower and sink is to make the contiguous and adjacent walls water-tight utilizing materials that resist being degraded by water, including hot water. Tiles made from ceramics are such materials and are basically inert to both the hot and cold water with which the tiles come into direct contact.
It is important also that the tile backer to which the tiles are adhered be water-resistant. Theoretically, it would seem that the water-resistant properties of the tile backer should be inconsequential because the backer is shielded from shower, bath and sink water by water-resistant tiles, grouting and mastic. However, experience has shown this is not the case and that moisture can and does in fact seep through the plies of material which overlie the tile backer. This can happen in various ways.
One way has to do with the fact that grouting is not water-impervious and over time permits the seepage of moisture, a situation which is aggravated upon the formation of cracks, including hairline cracks, in the grouting. Eventually, the moisture which penetrates through the grouting finds its way through the mastic and comes into contact with the paper facing of the wallboard. Such paper facing is typically a multi-ply paper, which upon contact with moisture tends to degrade by delaminating or otherwise deteriorating. For example, the paper facing is subject to biological degradation from mold and mildew. The paper can actually rot away. Furthermore, as the moisture comes into contact with the underlying set gypsum core, it tends to dissolve the set gypsum and also the core adhesive, which bonds the core and paper facing together. Such adhesive is typically a starch material. The development of these conditions can lead to tiles coming loose from the underlying deteriorated paper-faced gypsum wallboard. This undesirable situation is exacerbated when hot water comes into contact with the paper-faced wallboard.
Another type of moisture condition which leads to the loosening or falling off of tiles from their underlying support substrate is associated with those segments of the multi-ply wall structure which include a joint formed from an edge portion of the wallboard. An example is the joint formed by the edge of a wallboard panel and the lip of a bathtub. Another example is the joint formed by two contiguous wallboard panels. As moisture penetrates through the multi-ply structure and reaches such a joint, it tends to wet significant portions of the paper facing and core by virtue of its spreading through capillary action. This can lead to delamination of the paper facing and/or dissolution of the core and/or the paper/core adhesive. As this occurs, tiles can come loose and fall off.
The present invention relates to the provision of an improved gypsum-based structural panel of the type which can be used effectively as a tile backer and in other applications such as in the return air installations, shaft walls and area separator walls in commercial buildings where water and humid conditions are commonly encountered. Still other applications where moisture and humid conditions are likely to present difficulties with paper-faced gypsum board either during the installation or the use of the board will be apparent to those skilled in the art.
In efforts to mitigate or overcome problems associated with the use of paper-faced gypsum wallboard in applications where moisture exposure is expected to occur, the prior art has approached the problem in various ways over the years.
One approach to the problem has been to treat the paper comprising the facing of the wallboard with a water-resistant material sometimes referred to as a water-repellant. Polyethylene emulsion is an example of a material that is used to treat paper facing to impart water-resistant characteristics. Such treatment is designed to deter delamination of the multi-ply paper facing by reducing the tendency of the paper to absorb water which is a chief cause of delamination and to deter water from penetrating through the paper to the gypsum and destroying the bond between the paper-facing and gypsum core.
Another approach to the problem has involved incorporating into the formulation from which the gypsum core is made a material that functions to impart improved water-resistant properties to the set gypsum core itself. Such an additive tends to reduce the water-absorbing tendency of the core and decrease the solubility characteristics of the set gypsum. Wax-asphalt emulsions and wax emulsions are examples of such an additive.
Although improvements have been realized by the provision of gypsum wallboard prepared in accordance with these teachings, further improvements are still possible. Over a period of time, experience shows that tiles come loose from tile backer of boards having treated-paper facers, as the paper facing delaminates and the gypsum core erodes through the degrading action of moisture. The problem is particularly aggravated by warm water acting upon a gypsum core that includes either a wax emulsion or a wax-asphalt emulsion, commonly used, water-resistant core additives. While cores containing such materials have quite good water-resistant characteristics in the presence of water at room temperature, such characteristics start to fall off at temperatures in excess of 70xc2x0 F. and tend to disappear in the presence of water having a temperature of about 100xc2x0 F. or higher.
Still another approach to the problem is exemplified in commercially available structural panels comprising a Portland cement-based core sandwiched between facings of woven glass mat treated with a resinous material such as poly(vinyl chloride). The cement constituent of such products is more water-resistant than set gypsum, but such cement-based panels have a relatively high weight, and accordingly, are difficult to handle and expensive to ship. It is known to include expanded polystyrene in the cement-based core to reduce the weight, but even such lower weight panels are heavy enough to be unwieldy, weighing about 3000 to about 3500 lbs. per 1000 sq. ft.
In another approach, U.S. Pat. No. 4,647,496 discloses a structural panel comprising a water-resistant set gypsum core sandwiched between two porous fibrous mats. The preferred form of mat is described as a glass fiber mat formed from fiberglass filaments oriented in random pattern and bound together with a resin binder. Such panels differ from conventional gypsum wallboard in that the fibrous mat is substituted for paper as the facing materials of the gypsum core. Extensive outdoor testing has shown that glass mat-faced, water-resistant gypsum board of the type described in the aforementioned ""496 patent has much better weathering characteristics, including water-resistant characteristics, in outdoor applications than water-resistant gypsum board covered with water-resistant paper facing. However, prior evaluations conducted with such glass mat-faced board as a tile backer has revealed problems not unlike those encountered with the use of water-resistant board faced with water-resistant paper. Although glass mat has no tendency to delaminate like multi-ply paper, there is a tendency for moisture to dissolve and erode the gypsum of the glass mat-faced board. As this occurs, mastic with tile adhered thereto pulls away from the gypsum core. The loosened tile can eventually fall away from the wall.
Another more recent development in the water-resistant gypsum board field is described in U.S. Pat. No. 5,397,631. According to this patent, a fibrous mat-faced gypsum board is coated with a substantially humidity- and water-resistant resinous coating containing a latex polymer. The coating, which acts as both a liquid and vapor barrier, is formed from an aqueous coating composition comprising from about 15 to about 35 wt. % of resin solids, about 20 to about 65 wt. % of filler, and about 15 to about 45 wt. % of water, applied to obtain a solids loading of about 110 lbs. per 1000 sq. ft. A preferred resin for use according to this patent is a latex polymer which has been sold by Unocal Chemicals Division of Unocal Corporation under the mark 76 RES 1018. The resin is a styrene-acrylic copolymer that has a relatively low film-forming temperature. Coatings formed from the resin can be dried effectively at temperatures within the range of about 300xc2x0 to 400xc2x0 F. If desired, a coalescing agent can be used to lower the film-forming temperature of the resin. While this approach satisfactorily solves many of the previous-mentioned problems, the high cost of the resinous coating and the adverse impact that the coating has on the flame spread characteristics of the coated board has been an impediment to wider use.
The present invention is related to the provision of an improved, coated fibrous mat-faced gypsum board having a predominantly inorganic coating on the mat.
In accordance with the present invention, there is provided a moisture tolerant structural panel comprising a fibrous mat-faced gypsum board wherein the outer surface of the mat is coated with a coating which comprises a mineral pigment (pigmented filler material), an inorganic binder and a latex polymer binder. In particular, the coating comprises a dried (or cured) aqueous mixture of a mineral pigment; a first binder of a polymer latex adhesive and, a second binder of an inorganic adhesive. On a dry weight basis, the first polymer latex binder comprises no more than about 5.0% by weight of the coating, and the second inorganic binder comprises at least about 0.5% by weight, of the total weight of the coating. The second binder preferably comprises an inorganic compound such as calcium oxide, calcium silicate, calcium sulfate, magnesium oxychloride, magnesium oxysulfate, or aluminum hydroxide. In one embodiment, the second binder is included as an inherent component in the mineral pigment, as in the case wherein the mineral pigment includes aluminum trihydrate, calcium carbonate, calcium sulfate, magnesium oxide, or some clays and sands. The ratio, by weight, of the mineral pigment to the polymer latex adhesive in the coating is generally in excess of 15:1.